SCIENTIFIC GOALS
The capture of fault-zone guided waves would tell us about the structure of
fault zones, the healing of faults after a major quake, and the physical
nature of fault segmentation.

The Hector Mine earthquake, only the second M7+ quake in California in
decades and with a sterling surface exposure, is an excellent site for a
guided wave study.

FIRST VISIT TO FAULT
The installation was an adventure for the team of John Vidale, Paul Earle,
Fei Xu, and Hsi-Ping Liu, led by Yong-Gang Li (of USC, UCLA, and USGS Menlo
Park). After the requisite briefing on the multi-faceted hazards we might
encounter on the Marine Base, we discovered that none of our three cell
phones worked so far from LA. We could not make the hourly check-ins,
required to venture into the wilds. Unable to watch us lounge around any
longer, a sympathetic colonel assigned us a personal humvee escort, which
proved invaluable.

The road, impressive looking on the map, in some ways more resembled a
difficult path for a mountain goat. A mile short of the fault, the USC
Suburban was abandoned in favor of carrying the seismometers in the HumVee
and letting the scientists walk. The fault trace, once found, offset our
goat trail several times with several meters of offset, and thus was
clearly identifiable even to seismologists.

ARRAY LOCATION
An initial array was emplaced on Monday
October 18, in the Bullion
mountains, near 34 degrees 32.2 minutes N, 116 degrees 16.07 minute W. With
23 stations on a 250 m fault-crossing line, it is on fairly hard rock above
the region of greatest slip in the mainshock.

The fault zone at the site is 50-80 m wide, with one major and several
minor traces. The second trip on Friday October 22, with structural
geologists Gary Axen and Mike Taylor from UCLA, surveyed the region in
detail.

The fault zone was spanned with
a 100 m, 20 geophone line recorded by a Geometrix Stratoviewer. Although
the recorder was designed for high-frequency, active experiments, Yong-Gang
adapted it to trigger from aftershocks, with help from the LARSE
technicians in Northridge on Sunday morning. The line has L-22 2-Hz
3-component geophones. We are still translating this data into a
recognizable format, but captured more than 450 triggers between Monday and
Tuesday. At least half of the events appear to be aftershocks. All the
earthquakes listed on the SCEC Current Event website were recorded, and
presumably many smaller events as well. The disk was downloaded and cleared
to record more events on Friday.

Outboard of the dense line, we placed two L-4C's and another L-22, recorded
by a pair of REFTEK boxes. Each REFTEK recorded about 3000 triggers between
Monday and Friday, many of which are likely to be Marine bombing competing
with numerous small aftershocks.

We shown a pair of recordings
of the fault-parallel component of 10/20/99 15:17:45 (local time), M 3.3
aftershock. It's preliminary location is 34 32N 116 17W, with a depth of
2.2 km, or nearly under the array. However, the S-P time of 1 s indicates a
hypocentral distance of 7 km. The figure shows clear prolonged duration of
4-5 Hz fault-zone trapped wavetrain after the S arrival on the station at
the edge of the fault zone, but a much briefer S wave on the station
deployed 100 m farther away from the fault zone. It seems likely that this
event occurred near the main rupture plane of the Hector Mines mainshock
and excited fault-zone guided waves.

With the additional data from the dense line, and similar recordings from
the full array to be deployed in the next week, we should see a much
clearer picture of fault-zone trapped waves and hopefully of the internal
structure within faults from this experiment.